The spring of the Cherasca is situated in the southern part of the city, into sedimentary rocks formed by alternated sandstones and marls, dated back to 15-20 million years ago. The surface layer is composed by alluvial deposits (gravels, sand and clay) eroded from the underlying geological layer. The nature of these straits suggests anthropized phases alternated with alluvial ones. The lenticular bedding disposition of the deposits permits to uncover a sedimentary environment characterized by several channels that were activated during  important overflows.